Dave Coles, president of the Communications, Energy and Paperworkers Union of Canada, has alleged the three masked men had acted as agents provocateurs.
A video posted on YouTube from Monday's protests in Montebello, Que. shows Coles in a confrontation with the men.
He tells them to remove their masks. One of the three men is holding a rock and Coles tells him to move because their line is meant for peaceful protesters.
'These three guys are cops, everybody!' Coles can be heard shouting to the crowd as he tries to pull down their bandanas.
The three men then push their way into the police line and appear to be arrested, then taken away.
Police said that after viewing the clip, they were able to confirm the men were Quebec provincial police officers.
Photographs taken by another protester had shown the three men lying on the ground with the soles of their boots adorned by yellow octagons. A police officer kneeling beside the men appeared to have the same imprint on his boot -- likely the Vibram boots logo.